During the production of paper pulp, the fibre material is repeatedly diluted and dewatered in dependence on its treatment in different stages of the process. Many different devices, such as suction filters, screw presses, roller presses and centrifuges, have been developed for this purpose. To achieve high dry substance contents of up to 30% requires large forces and hence large and power-consuming devices. A method which is often used for dewatering to such high dry substance contents consists in centrifuging the pulp suspension. The device often includes a liquid-permeable rotating basket in which the centrifugal force drives the suspension out towards the inner wall of the basket and presses the liquid through the wall. For the system to function satisfactorily, the suspension has, on the one hand, to be dispersed across the basket and, on the other hand, to be transported onwards during and after the dewatering; it is also necessary to ensure that the liquid-permeable basket is not plugged by fibres. It is a common practice to arrange a screw inside the screen basket for the purpose of dispersing and transporting the suspension. However, when a cylindrical screen basket is employed, substantial forces are required to move the fibre mass. Furthermore, there is a tendency for the fibre mass to collect against the screw blade during transport, resulting in the dewatering being impaired.
Another problem is that the suspension is difficult to set in rotation in the screen basket since it is only the inner wall of the screen basket which transmits the rotation to the suspension. The transmission of energy is poor in the case of a pulp suspension, which contains large quantities of water, indicating that the liquid will only very slowly begin to rotate inside the screen basket, irrespective of screen speed, and will require a long time before it gains a centrifugal force which is adequate to ensure efficient dewatering. The plugging of the screen basket is an additional problem; attempts to solve this make use of different elements such as, for example, nozzles which act from the outside and blow either air or liquid against the screen basket.